High-resolution imaging captures cavity-induced density waves in a quantum gas

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A new study, published in Physical Review Letters, reports that scientists have successfully imaged the formation of cavity-induced density waves induced by laser light in an ultracold quantum gas. Previously, only global signals, such as photon leakage or the peak in energy deposition of a fast charged particle (Bragg peaks), have been used to detect this kind of ordering. Prior to this study, there had been no direct, high-resolution in situ imaging of cavity-induced density-wave order in ultracold gases.

Lasers, optical cavities, and unitary Fermi gases

When laser light is arranged so that it bounces back and forth between two mirrors, light waves become trapped and create what is referred to as an optical cavity. This creates standing waves or amplifies light through resonance. When atoms in an ultracold unitary Fermi gas are placed in an optical cavity, they can absorb and emit this light. Unitary Fermi gases exist in a strongly interacting state where the wave scattering length makes interactions independent of the specific atomic details.

Light emitted by atoms in the gas can be absorbed by other atoms. This exchange of photons creates further interactions between the atoms that can cause a self-rearrangement into a periodic pattern within the gas, referred to as a density wave. This self-organization occurs above a critical threshold, called the superradiant phase transition, where the exchange of photons enables simultaneous, collective interaction among all atoms.

"Photon exchanges between atoms within the cavity mode produce a tunable long-range interaction, which can reach

large strength, overcoming other energy scales such as the one given by temperature, kinetic energy, potential energy, atom-atom repulsion, or Fermi pressure to yield a superradiant phase. Unique to this platform, photons leaking from the cavity provide real-time, weakly destructive information about the dynamics of the ordering process, allowing for detailed investigations of order formation and dynamics. Superradiance is accompanied by spatial ordering of the atoms, which can be detected as Bragg peaks using standard time-of-flight images," explain the authors of the new study.

Watching the formation of a density wave

To image the formation of the density waves in the gas, the research team designed a high-numerical-aperture microscope that combined real-time photon detection with high-resolution absorption imaging—a technique in which the reduction in intensity of a probe light beam is measured as it passes through a sample. They first illuminated the gas with a pump laser in the cavity to the superradiant phase. The team then used absorption imaging, which they say enables analysis of density correlations throughout the ordering process, and during ordering at higher-order cavity modes.

The study authors write, "The combination of single-shot imaging with single trajectory readout of the cavity photons allowed us to reconstruct atom-field correlations, testing for the first time the foundations of cavity-induced interactions."

The team says they observed long-range spatial correlations and could track the formation and dynamics of density waves in real time. They found that the spatial pattern is controlled by the cavity mode structure, and both atomic and photonic observables are strongly correlated. Imaging also revealed a uniform phase and amplitude of the density wave across the cloud, which confirms infinite-range interactions.

This new type of microscopy could be extended to image magnetization or pairing patterns in more complex quantum states. It also enables the engineering of interaction patterns and the exploration of new quantum phases.

Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

Tabea Bühler et al, Microscopy of Cavity-Induced Density-Wave Ordering in Ultracold Gases, Physical Review Letters (2026). DOI: 10.1103/h3zm-rnnx. On arXiv: DOI: 10.48550/arxiv.2511.08510

Journal information: Physical Review Letters , arXiv

Key concepts

Cold atoms & matter wavesOptics & lasersPhysics & societyQuantum fluids & solidsQuantum many-body systemsStrongly correlated systems

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